This invention relates to a novel electrophotographic film and to a photoconductive layer or coating comprising one of the components of the said electrophotographic film.
The photoconductive material which forms the coating is produced by a special sputtering process to achieve the properties which will be detailed below. In the case of the electrophotographic film, the coating is laid down in a thin film layer on a substrate which is preferably transparent and flexible, with an ohmic layer interposed between the substrate and the photoconductive thin film layer or coating. The photoconductive coating will be specifically described hereinafter as cadmium sulfide although zinc sulfide and other substances discussed in detail are useful as photoconductive thin film coatings of a similar nature when applied according to the invention.
The expression "thin film" is used herein, both in the specification and in the claims. As a general rule the scientific literature in some way attempts to define thin film in terms of the properties of the substance being discussed, calling attention to those properties in contrast to the properties of the same substance in bulk. This latter is called "bulk properties" herein. Speaking in relatively simple terms, some materials act differently when constituted as a "skin" than they do as a "body." Reference may be had, for example, to a publication entitled "Thin Films" by Leaver & Chapman, Wykeham Publications (London) Ltd., 1971 for a general discussion of the differences between thin film and bulk properties of the same type of material. In that publication, the thickness of a "thin film" is given as "usually less than one micron." This general definition is required in view of the breadth of the subject covered in that publication.
When one considers the purposes and requirements of the structures in which a certain category of material is to be used, the boundary or boundaries between the thin film and the bulk properties must take these purposes and requirements into consideration. Properties which are of no importance or interest to the solution of a problem do not enter into the matter and hence should not establish the physical criteria. For example, if a radical change in the sound reflecting property of a certain material occurs when the material is made about 2 microns thick and less because of skin effect, then if that material is going to be used in an environment which uses the sound reflecting property, it is exhibiting a thin film effect. On the other hand, if that identical material changes its resistivity radically only when its thickness is decreased to 0.5 microns or less then, for the conditions of use in which its resistivity is of importance, the material is still a bulk material at thickness greater than about 0.5 micron.
The use of the materials involved herein relate to several properties which are beneficial and advantageous for the invention, and the meaning of the expression "thin film" as used herein will be related only to these properties, irrespective of the properties of any other materials for any other purposes which may have been referred to as thin films in thicknesses other than those which will be defined. The words "thin film" when used in the specification and in the claims will be taken to mean a thickness at which the desired properties of the material in question cease acting as bulk properties and commence acting as a skin or thin film. Examples which have actually been made is substantially less than a micron (10,000 Angstroms) and very few of the coatings or layers tested exceed 5,000 Angstroms. Accordingly, a thin film will be considered one that is substantially less than a micron thick.
Considering the prime example of the photoconductive coating herein, namely pure cadmium sulfide, the anisotropy which is essential to the invention is exhibited in all examples which were made, none being much over 6,000 Angstroms thick. The transmissivity requirements of the invention which relates to the ability of the photoconductor to absorb photons were optimum (between 70% and 85%) when the coating was between 8500 and 2500 Angstroms in thickness. The high dark resistivity of the film occurred only when the thickness of the film was below 6000 Angstroms.
With respect to the thin film ohmic layer, which will be described, its desired properties were conductivity, bonding ability and transparency such that when a desirable photoconductive coating is carried thereon the total light transmissivity of the electrophotographic film will be 70% and 85%. This was achieved by a layer several hundred to about 500 Angstroms.
The total thickness of both the photoconductive and the ohmic layers on typical electrophotographic films ranged from 3500 to 5000 A or 0.3 to 0.5.mu..
The expression "photoelectric gain" as used herein has a meaning requiring explanation. The speed and efficiency of an electrophotographic member is directly related to the "hole-electron pairs" produced when subjected to light. In prior art photoconductive coatings used in xerography or electrofax, it requires many photons (extremely bright light) to produce a single hole-electron pair. The number is usually upwards of a thousand. It follows that if electrophotographic film such as that of the invention can produce a hole-electron pair upon the incidence of a single photon or even two 1/2 photons its "photoelectric gain" is very substantially greater than that of the prior art. Accordingly, in order to provide an expression for the gain of the invention, "high photoelectric gain" will be intended to mean a condition in which it requires at most one photon to produce a single hole-electron pair.
Reference will also be made hereinafter to "high speed". The criteria of prior art electrophotographic members furnishes no real comparison since the speed of the electrophotographic film of the invention is so much faster than the speed at commercial members. In fact there is no commercially available electrophotographic transparent film.
Attempts to establish a comparison of speed between the electrophotographic film of the invention and ordinary photographic film are at most qualitative. When charged, the film of the invention can be exposed at high speeds such as of the order of 0.01 second. Since A.S.A. ratings have been devised for negative film of the silver halide type and take into account contrast, fogging and other photographic factors they really don't apply here. Suffice it to say that exposures can be made at speeds comparable to those of silver film. Doping can increase speed substantially.
The expression "electrophotographic film" or "photographic film" as used herein is intended to mean a complete article with several layers or lamina for use in some photographic process. Reference to the substrate or substrate member or substrate means will not include the use of the word "film" although the substrate which is contemplated by the invention could be considered a film in the ordinary meaning of the word. As will be seen, it is preferred that the substrate be a thin flexible transparent member of plastic sheeting, commonly known as plastic film.
In electrophotographic processes as known, an electrostatic latent image is formed on the surface of a photoconductive member. The photoconductive member is initially charged over its entire surface while in the dark, the charge being retained, if at all, for a period of time which is primarily dependent upon the physical character of the material or materials from which the photoconductive member is made. Immediately after the surface has been charged, such surface is exposed to some form of radiant energy comprising a pattern of tones, lines, text and the like which it is desired to reproduce. Such radiant energy may take the form of a projected light pattern, an X-ray projection, etc.
The areas of the surface of the photoconductive member which are exposed to the brighter portions of the pattern become more conductive than those which are exposed to the less illuminated portions of the pattern. There is a conductive member immediately below the photoconductive member which forms the ground plate to enable charging the photoconductive member initially. The selective conductivity of the illuminated portions of the photoconductive member selectively and proportionally discharge the electric charge from the different areas of the surface of the photoconductive material in accordance with the respective degree of illumination thereof.
Not all materials will accept a charge initially, and of those which accept a charge many will immediately leak off so that even without illumination the charge decays so rapidly that it is almost useless. The application of toner demands charge retention.
In addition to accepting a charge and retaining it in darkness, the photoconductive layer is required to discharge in the light areas to a degree which is fairly rapid and generally proportional to the amount of impinging light. The amount of discharge of a photoconductive member is a measure of its gain, that is (as defined above) the number of photons which will be required to discharge a surface electron and the ability of the member to permit the recombination of the hole electron pairs. In prior art members, and where charge at the surface is positive a conductive layer is needed to aid in discharge, but, as will be seen, this is not the case with the preferred embodiments of the invention. Most known electrophotographic members such as the xerographic plates and the electrofax sheets popularly used have very low photoelectric gain. To be comparable with the silver halide emulsion films of today, gains would have to be enormously greater than those of these prior art photoconductors, but until the advent of the invention this has not been accomplished so far as known. The rate of discharge of a member for a given light intensity when exposed is a measure of the speed of the photoconductive member. This rate is measured in seconds and at best fractions of seconds in known photoconductive members. As will be obvious, modern high speed photographic films can be exposed in milliseconds. As will be seen this latter is also practical with the photoconductive material of the invention.
Another aspect of photographic films which finds little counterpart in most electrostatic processes used for reproduction or copying purposes is the continuous tone gray scale. The dark parts of the pattern are represented by the retention of charge and the light parts are represented by discharge. Known electrostatic members cannot discharge fully, even in the brightest of light. The charge retention properties are not good enough to provide the dense black that most photographic films can achieve.
continuing with the discussion of known electrophotographic processes, the resulting geometric pattern of charge on the surface of the electrophotographic member, whether in xerography or electrofax, constitutes the latent image mentioned above. The charge, if sufficient in potential, has the property of being able to attract fine particles suitably polarized, electrostatically. In the art of xerography and electrofax, such fine particles in the form of a comminuted pigmented powder or liquid suspension thereof are brought into contact with the surface. The particles selectively adhere to the surface in varying degrees according to the charge pattern represented by the latent image, following which the excess is brushed or otherwise removed from the surface and the remaining toner, as the particles are called, forms a visible image. In xerography, this visible toner image is transferred to a receiving element such as a sheet of paper and is permanently fused or "burned" to the surface of the receiving element by techniques which are well known. In electrofax there is no transfer, the image being fused to the electrophotographic member and this latter becomes the copy.
The problems alluded to arise in connection with providing a tonable image. There must be sufficient surface potential to result in enough charge to attract the particles; there must be sufficient charge to provide dark increments where the image is dark; the charge must remain in place and not leak off in a period of time that the mechanical problem of bringing toner to the surface, and if effected, or transfer or fusing occurs. In this latter respect, even if there were an excellent image produced by the charge and toner (assuming that the toner is not self-adhering) but the charge leaked off to a substantial extent in the period of time required to bring the toned image into engagement with the transfer sheet or into juxtaposition with the fusing apparatus, then a large portion of the toner would have dropped off. The image would be light or spotted.
The xerographic and electrofax processes as known are not readily adaptable to the same purposes as present day photography. Further, the inherent characteristics of these processes proscribe their likelihood of ever being useful in high speed photography. The most familiar xerographic process of the present time utilizes a large metal drum coated with amorphous selenium as the photoconductive member. The photoconductive member has extremely low gain and is very thick -- of the order of a fraction of an inch -- in order to be able to build up a sufficient charge to enable toning. Low surface potnetials during charging require longer toning times. The process performed is complex, occurs in a complicated and expensive machine, and the speeds, resolution and flexibility of such machines and the processes thereof leave much to be desired. Electrofax equipments of the present time utilize zinc oxide coated conductive paper which is charged, exposed, led through a toner bath and fused. The photoconductive gain is again low, the resolution crude, the gray scale short and limited, the equipment complex and bulky.
Neither of the processes of the prior art described is capable of being embodied in a small hand-held camera, and even if this could be done, neither process nor any other known process is capable of approaching the speed and quality achieved by the ordinary high speed camera using fine grain silver halide emulsion photographic film. The invention has this capability.
Inherent faults with the known methods, apparatus and the photoconductive materials and articles used have prevented use in such fields as high speed photography, fine resolution microphotography, and many other technical areas. Record-keeping, by means of projectable microfilm is a field wherein there is a long-felt need for a process for making the photographic member quickly, with high resolution, economically, with simple apparatus and having the ability to withstand long periods of storage. For example, it would be highly desirable to add information to a microfilm record from time to time without adversely affecting the information which is already contained thereon.
Conventional photographic microfilm is not capable of being re-exposed for adding information. The inherent construction and processing thereof causes an irreversible chemical change when the microfilm is developed. The general electrophotographic process above-described could provide a suitable microfilm record if it could be used to make a transparent electrophotographic film having high resolution and prolonged storage life. As can be seen, if the photoconductive coating of such electrophotographic film could be preserved indefinitely, then each time that an addition is to be made to the record already contained on the coating, one merely charges the surface of the coating, exposes the same, and fixes the new image to the surface. This presumes that toner is applied directly to the surface and fused to the surface permanently and that the film is transparent.
Known experimental transparent electrophotographic recording elements are susceptible to deterioration on prolonged exposure to light, elevated temperatures and humidity. They must be handled carefully, stored under controlled conditions and can be re-exposed only a limited number of times. Their use for records of a permanent nature is highly limited. Accordingly, it is impractical to utilize the same for such records. None is known to be commercially available.
The above discussion considers only a limited aspect of the prior art deficiencies. A consideration of some of the problems solved by the invention will emphasize that the advance in the art by such invention is not confined to a small area.
The conventional silver halide gelatine coatings of photographic film achieve greater speed and better resolution than known electrophotographic members of the so-called xerographic and electrofax type. Nevertheless, such gelatin emulsions are subject to disadvantages which are obviated by the invention, in addition to the fact that the electrophotographic film of the invention can be repeatedly exposed to add information to the same without deleterious effects.
The conventional silver halide film of 140 microns thickness has an emulsion which is about 20 microns thick. The thin film photoconductive coating of the article of the invention is a fraction of a micron thick. The conventional silver halide film is thus not easily flexed without damage. Its resolution is determined by the size of the silver grains; the bigger the grain, the faster the film. In production, the film cannot be inspected in ordinary light, it cannot be handled or transported except in special dark packages. The emulsion is soluble in ordinary liquids and is hygroscopic.
The electrophotographic film of the invention, on the other hand, is highly durable. Its thin film coating is extremely dense and hard as glass; insoluble in most liquids; has extremely fine resolution; is produced by sputtering processes in pressure vessels and hence is free of bubbles; is not affected by light and hence can be handled freely and readily inspected under bright light. Because it is a semi-conductor, it is nonhygroscopic and not subject to deterioration on account of any of the factors which will deteriorate the ordinary silver emulsion type of photographic coating. Fungus or other microorganisms have no effect on the electrophotographic film of the invention.
The ordinary unsensitized photographic emulsion and such electrophotographic coatings as known have a relatively limited spectral response. The photoelectric gain of known electrophotographic coatings is substantially less than that of the article of the invention which accounts to a large extent for the inability of prior electrophotographic films to have the extremely high speed of the invention.
The photoconductive coating of the invention is independently novel because it exhibits properties which have not been known in such substances. These properties will be detailed hereinafter, but for the moment, the coating is a thin film, extremely dense, highly ordered microcrystalline, wholly inorganic, has extremely high photoconductive gain, has ability to accept extremely high per unit surface potentials, retains charge in darkness for periods of time enabling complete toning, is extremely light sensitive and is substantially panchromatic. It is applied by a sputtering method using radio frequency sputtering equipment, but the method is modified by establishing a second dark space adjacent the anode in addition to the normal cathodic dark space which produces the extremely dense highly ordered crystalline deposits.
The photoconductive materials known have some degree of photoconductive persistence which is annoying since it prevents high speed exposure and sharp images. When subjected to light, the discharge of the photoconductive coating commences, but when the light is cut off, most prior art coatings continue to discharge for some time. In the coating of the invention there is no photoconductive persistence when the exposure is completed. The discharge immediately is cut off when the coating is placed in darkness, giving the high speed and high resolution images described herein.